The purpose of this article is to list some of the common problems encountered with humidity control in modern high-speed air conditioning. It will explain how controlling blower speed from a temperature signal will eliminate these common problems as well as increase the comfort and livability of.a home.

In the quest for energy efficient operation, the residential HVAC industry has adopted practices that are based on opinion rather than fact. The consequences of some of these practices have resulted in major discomforts for the homeowner. Most homeowners will adopt measures to circumvent these discomforts. An example of this is that the homeowner will reduce air conditioning thermostat settings by about three degrees to get away from that muggy feeling. This requires about an additional 18% in energy consumed by the air conditioner. So, what looked good to energy geeks who don’t get out of the lab and don’t have a clue as to the overall function of an HVAC system, has been eaten-up by the homeowner’s real-world desire for comfort. .


  1. If water stands in a drain pan, the pan is either poorly designed or improperly installed.
  2. What is normally called “low load conditions” refers to load from temperature gain and is not necessarily the total load necessary for efficient operation and comfort.
  3. Humidity can be a significant load that is not addressed.
  4. Every 10% drop in relative humidity is the comfort equivalent of a 3° drop in temperature. I.e. drop the humidity by 10% and you can enjoy the same level of comfort at a three-degree higher thermostat setting. This saves energy.
  5. Water adheres to the A/C coil better as the coil gets colder.
  6. When the A/C compressor turns off and the blower’s speed remains high, the coil gets warm very quickly and doesn’t grip the water as tightly.
  7. The faster that air moves over an A/C coil, the higher the coil’s bypass factor, the warmer the coil, the warmer the delivered air temperature and the less adhesion to water.
  8. A warm coil (either right after the compressor shuts off or under high air velocity) doesn’t hold water tight enough to give it time to flow down the entire height of the coil to the drain pan before being blown from the coil by the high-speed air movement through the coil.
  9. If (with the compressor running) the air velocity over the coil is less than 500 feet per minute: An A/C coil, under normal operating conditions, will hold water long enough for it to flow down the coil and into the drain pan.
  10. The method of determining the air velocity (called face velocity) over the coil is to divide the CFM air delivery from the blower by the square foot face area of the coil. When velocities reach over 600 feet per minute, then water will definitely strip from the coil and down the ducts even if the coil is quite cold.
    1. The space between the coil plates or fins (Usually 14 fins per inch) is designed to allow air to move freely through the coil and still allow enough air to impact the coil to reduce the temperature of the air enough for effective cooling and humidity removal.
    2. The thickness of the coil. Usually allows 3 or 4 rows of tubing.
    3. Water that is flowing down the coil takes up space. There is a condition where the amount of water in the coil can build up because it can’t get down to the drain pan fast enough, this excess water takes up free space and reduces the amount of free area for air to flow at an efficient (500 ft. per minute) velocity through the coil. It has the same effect as reducing the size of the coil. In this case, the air velocity through the remaining, or available, free area increases to over 600 feet per minute. At that point water is stripped from the coil.
  12. Coil bypass factor is the percentage of air that goes through a coil without being affected by the coil. Cooling is done only to the air that actually touches the coil’s fins.
  13. Typical bypass factors for a three-row coil with 14 fins per inch are:

















The delivered air temperature is the result of mixing very cold air cooled by the coil fins and the room temperature air that goes through the coil space without being cooled.


Blower speed timed programs:  i.e. run the fan for a number of minutes (maybe 7 minutes) on low (about ½ speed) During this time the coil is super cold and super efficient and collects large amounts of water. During certain high humidity conditions, the A/C coil loads up with water faster than it will drain. After the low speed segment has timed-out, the blower ramps to high speed. This does two things: First it warms the coil, which reduces the water’s adhesion to the coil. Second, high velocity through the remaining free space breaks the surface tension of the water so the droplets break apart and they blow off the overloaded coil and down the ducts. This is why many contractors in high humidity areas do not use this feature.



In high humidity areas, it is also common to either run the fan on intermittent or turn off the fan for a period of time after the compressor shuts down in an attempt to give the coil time to drain. This is not a solution to the recognized problem of blowing water off the coil and re-evaporation. It is avoiding the problem rather than solving the problem. It also makes the home less comfortable, less quiet and the wide variety of indoor air quality products less efficient.


The common statement concerning coil drain pans is: “When you run the blower while the compressor is off, you’ll re-evaporate the water standing in the drain pan.” Somehow this statement has been used to the point that it has become folklore. The proper design and tilt of the drain pan is all that is required for it’s proper function. First, the word drain in the name of the device should provide a clue to its function. Water should not be standing in a drain pan. Second, to evaporate a couple of ounces of water at temperatures we are dealing with is not instantaneous. Third, even if a few ounces of water were evaporated into the thousands of cubic feet of air that are inside a home, it would have about a zero consequence.





  1. Blower speed can be controlled effectively and efficiently by changing the voltage to any high quality shaded pole or PSC blower motor. It has been done since the late 1950’s.
  2. When blower speeds are controlled through quality motors, the fan laws are in full effect.
    1. When you reduce the speed of the blower by 50%, you reduce the CFM by 50%
    2. When you reduce the speed of the blower by 50%, you reduce the static pressure to one fourth.
    3. When you reduce the speed of the blower by 50%, you reduce the power required to one eighth of that required to run it at full speed. (i.e. You can run a good blower motor for eight hours at half speed for the same amount of money that it would take to run it full speed for one hour)
  3. Typical speed/temperature relationships for blower speeds controlled by temperature. Using a typical, average designd for a three-ton system. (Using rounded numbers for simplicity)
    1. 36,000 BTUH
    2. 400 CFM per ton or 1,200 CFM at full speed
    3. Minimum speed is set at about 35 or 40% of full speed or about 450 CFM at about 450 rpm blower speed when the delivered air temperature coming off the coil is above 68° F.
    4. Maximum blower speed about 1070 RPM when the delivered air temperature coming off the coil is about 50° F.
    5. The fan is set to run continuously.
    6. For every degree of temperature reduction from 68° F delivered air the fan motor’s speed increases about 34 RPM. Or for every 34th of a degree temperature drop, the fan speed increases one RPM. This is smooth and seamless.




  1. Room temperature about 75°.
  2. Indoor humidity high. (maybe over 70%)
  3. Blower running continuous at about 450 RPM and delivering about 450 CFM.
  4. Compressor comes on.
  5. Air is moving over the coil slowly.
  6. The bypass factor is about 10%, which means the coil is super efficient and very cold.
  7. Because the coil is very cold and super efficient, it cuts into the latent load and begins rapidly collecting water.
  8. Because the coil is so cold, the air coming across the coil also cools down rapidly and reduces the delivered air temperature.
  9. Because the air temperature is reduced, the blower’s speed is automatically and instantly increased.
  10. Here are some interesting points: The coil has established its grip on the latent load and is collecting water to the coil’s maximum design capability to capture water.
  11. The compressor doesn’t know the difference between latent and sensible loads. It only sees a load.
  12.  Because the coil is capturing water to it’s maximum ability, the remaining capacity of the compressor is then devoted to the sensible load. The remaining compressor capacity may not be enough to drive delivered air temperature down to the 50° temperature required for full speed blower operation. The delivered air temperature might be 54° degrees and between 900 and 1,000 CFM. (That is why when Florida, gulf coast and contractors in other high humidity areas first install temperature controlled blower modulating controls, they think that the blower is going too slow. However, when they look at their gauges, they see the unit is working its lungs out because it is concentrating on the latent load. A day or two later, the house has dried out. The delivered air temperature is colder and the blower is running faster.)
  13.  Compressor Shuts off:
  14.  The coil and delivered air begin to warm a little. The blower’s speed immediately and smoothly drops to about 50% of the way between the highest speed that it achieved at its coldest point and the 450 CFM minimum speed. Lets say about 675 CFM.
  15.  675 CFM isn’t going to blow water off a 1,200 CFM coil. The coils is going to drain.
  16. Interesting point #2: There is a good amount of cooling capacity in pressure equalization. The high-pressure liquid refrigerant moves into the evaporator and does some cooling. It usually takes about 5 to 10 minutes for the pressure in a system, with good valves, to equalize.
  17.  During this time, the blower’s speed is smoothly dropping towards the minimum speed. The coil is cool, the coil is draining, and there is no re-evaporation.


Temperature controlled blower speed:

  1. Eliminates or greatly reduces the problem of blowing water off A/C coils.
  2. Results in maximum humidity removal by automatically adjusting for latent loads.
  3. Allows comfortable and quiet continuous blower operation that permits all accessories attached to an HVAC system to operate at full-time at full efficiency.
  4. Increases efficiency.

Dick Peters, P.E./CM


FanHandler Inc.


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Humidity and mold caused by zoning bypass.


I want to pick your brain as you are one of the few people I know who understand the question.

Danny Gough (Housedoc) HVAC Done right!       


Hi Danny. It’s great to hear from a good friend and very valued long-time customer.


(Bold type are my opinions, Regular type is by Danny.)

Thanks Danny for referencing my white paper on


Everything in there is dead-on true. The part that you mention concerning the colder the coil the greater the adhesion between the coil and water. That is true, but not an enormous difference. The 500 ft. per minute face velocity remains the threshold air velocity, above which water will be stripped off the coil and down the duct. I don’t know of an exception. Skipping ahead a bit: The fact that you are seeing mold 12 feet down the ductwork strikes me as caused by a ridiculously high-speed fan combined with the zone bypass. This combination is responsible for increased air speed stripping water from the cooling coil.


One VERY bad one is  stachybotrys chartarum


Yes, we are seeing and hearing of mold becoming an increasing problem in residential HVAC system’s. The HVAC industry’s archaic infatuation with switches and high-single and two-speed blower motors has a lot to do with mold growth in the ductwork. Our self-adjusting, modulating Whisperer 4-Z blower speed controls eliminate bypass mold problems by eliminating the bypass. The bypass attempts to control duct pressure by short cycling excess air from the supply plenum to the return. A FanHandler Whisperer 4-Z control adjusts the duct pressure by adjusting the blower’s speed to hold your pressure setting. Once set, the pressure will hold within 0.02” w.c..   

I am seeing an epidemic of mold inside air handler cabinets, in supply plenums and in the first 10 to 12 feet or so of supply air trunk.

mold in duct

The fact that the mold is being spread 10 to 12 feet down the ductwork points to way too much air going though the evaporator coil. Also, that this mold has been around long enough to cause considerable damage to the HVAC system. Hopefully it is caught before causing significant health problems. If I were on that job, I’d inform the homeowner that you have discovered an almost certain health hazard. And whoever is responsible for this mess had better immediately contract with a mold remediating company. If folks get sick from the mold, the responsible party better have super deep pockets or a lot of insurance. What is not clear to me, is: IF you discover mold and do not inform the homeowner, and/or health authorities; might you, in some way, share responsibility for subsequent problems. That could be devastating!


There are three common denominators that accompany an almost 100% occurrence. 

  1. The air handler cabinet and supply duct is insulated internally with fiberglass duct liner. I do not see visible mold in bare sheet metal ducts. But air handler cabinets that are lined with fiberglass are covered with mold.

Interesting that you don’t get mold on the sheet metal.

  1. In addition to duct liner, the second factor that seems to guarantee mold inside the cabinet and supply duct is ZONING. In particular, zoning that includes a bypass damper, either barometric or electronic.

Bypass dampers are the Achilles heel of zoned systems. There are many HVAC contractors who WILL NOT sell zoned systems because of associated bypass problems. Any FanHandler Whisperer 4-Z eliminates the bypass and its problems by eliminating the bypass and controlling the duct pressure by the blower’s speed.

  1. Every instance I have seen thus far is on singe speed compressor systems. The blower motor doesn’t seem to matter. I have seen mold develop with both fully variable speed and X13 fixed torque motors.

Fully variable speed and the fixed torque motors are often controlled by a time/speed formula. When the speed increases with time, it just delays by a couple minutes, when the blower will strip water from the coil. Nothing tells the motor to slow down when it should.

bypass duct

The bypass damper is simply a cheap way to handle excess air when a small zone is calling that cannot handle full airflow. All this does is relieve excess air by recycling it from the supply plenum right back into the return. The more the system runs with the bypass open, the colder the return air temperature gets. I muse that the colder the return air temperature gets, the colder the evaporator coil. The colder the evaporator coil, the more moisture forms on the coil. 


That is a great explanation of what’s happening within the equipment! Now add the facts that once the bypass damper opens, you introduce another problem that further increases air velocity through the coil. The blower in the furnace section is creating a high pressure between the blower and the coil. Added to that, the bypass is contributing a suction on the outlet side of the coil. High pressure on the inlet side of the coil and low pressure on the leaving side of the coil increases velocity through the coil. Thus, more water, more mold, more mold spores and more trouble. Once mold matures and sheds spores, these spores are blown out of the HVAC system into the home. Then into the most sensitive and efficient filters ever produced; somebody’s LUNGS!


In most cases, there is a temperature sensor in the supply plenum that shuts off the compressor when the discharge air temperature drops to 40 degrees or so. Although the compressor shuts off, the indoor blower continues to run until the evaporator coil warms up above 40 degrees. 


In some cases, the discharge air sensor is omitted. In those situations, the compressor is allowed to continue running, likely freezing the coil in extreme situations. 


I am trying to uncover the correlation and possibly the cause of the mold as it relates to this bypass situation. That is where I am hoping you can enlighten me.


You’ve given us plenty of valuable information. I’ll try to expand on it.


Taking another look at your equipment drawing it is easy to recognize that when any of the four zone dampers begins positioning towards closed, the air pressure past the evaporator coil increases. This pressure opens the barometric bypass damper. Once a barometric bypass cracks open, we may as well accept that it is virtually all the way open. The air is now screaming through the bypass duct. It is way exceeding the 500 feet per minute water stripping threshold of the evaporator coil. Resulting in a lot of water in the ductwork.


FanHandler Inc. manufactures way advanced electronic motor speed controls especially designed for HVAC comfort and efficiency. During periods when there is no call for heating or cooling, the blowers speed can be set to the point where you can’t hear air movement or feel it a foot from any register.


The HVAC system can be any type (forced air furnace, heat pump, fan/coil unit.) New or 15 years old. Any equipment in reasonable condition

Can be modernized to provide unmatched comfort and efficiency.


For best results you might want to set the minimum speed at about say ¼ speed. At ¼ speed, fans will run for 64 hours for the same energy cost as at full speed for one hour. You won’t feel it nor hear it. So why run it at all?

Because when air circulates through a home quietly, smoothly, imperceptibly and efficiently, It gently pulls air from the sunny side of the home brings it back to the HVAC equipment where it is mixed with air from the cooler areas. All rooms stay fresh and one temperature. The entire HVAC system, including all Indoor Air Quality equipment operates at peak efficiency.


The stakes are much higher in NC because, trying to solve a comfort problem, the licensing board for HVAC contractors requires zoning on all two-story houses.





"Water adheres to the A/C coil better as the coil gets colder.”


When I was a student at the University of Minnesota, I lucked out and got a job as a machinist in the Mechanical Engineering Research Machine Shop. Our mission was to build anything that the professors needed in their experiments and project for their Doctorates. One of the professors I worked with had solar panel heat collection. I built a solar panel out of a 4’ x 8’ sheet of copper. I brazed a copper tubing line that snaked back and forth across the sheet. Then added a smaller side panel that had no contact with the solar panel and could be set at any angle and direction as possible. We ran cold water through the tubing and got an amazing amount of hot water out the other end. Once that was working as best we could make it, he experimented with slanting the solar panel at different angles to work out a formula for effectiveness at various sun angles. As a sidelight he placed a drop of water on the small side panel at every angle and measured the temperature of the panel and time it took the drop of water to run down hill one foot. The colder the panel and the closer to freezing, the slower the water would run. Therefore, the colder the coil the more adhesion to the copper. Not a huge difference but a definite measurable difference.


In this bypass situation, we definitely have a cold coil. But I am trying to put the pieces together. After this happens I want to understand the connection between cold coils, saturated with water and mold. I just cannot figure out what’s happening, but I have a lot of questions trying to sort it out.

Does the coil being overtly saturated with moisture increase the bypass factor? I’d agree with that. Is there a point where the bypass factor increases the face velocity through the coil and you get water blowoff? The face velocity is a set number as explained below..



Coil Bypass Factor is the percentage measurement of the air that travels through the coil that is not affected by the coil. It bypasses (doesn't touch) any cold surfaces of the fins, tubes, etc. This means that the discharge air from the coil is actually a mixture of air that is warm return air plus conditioned air that is quite a bit colder. The bypass factor tells us the percentage of return air that makes its way through the coil without changing temperature. The Face Velocity of a coil is the cfm that the blower is putting out divided by the square foot area of the coil when looking at it strait on. Here are four examples of the bypass factor for a (I think this coil description is correct, I grabbed it out of some old work I was doing) (Dry Coil) 3 tube coil with 14 fins per inch. Face Velocity (FV) 200 ft./min = (BPF) 0.07 which means that only 7/100 or (7%) of the return air is not cooled. (FV) 300 ft/min = (BPF) 0.11 or (11%) is not cooled. (FV) 400 ft/min = 0.14 or (14%) is not cooled. (FV) 0f 500 ft/min = (BPF) of 0.18 or (18%) is not cooled. Finally the fourth fundamental: Is somewhat rule of thumb and is that when the face velocity reaches over 500 ft/min, water is ejected from the coil and is not caught by the drain pan.  A couple reasons for this is that the faster the air, the warmer the coil. The warmer the coil, the less adhesion it has for holding onto the condensate. So unnecessarily high speed air (Whether caused by a high speed fan or a zone bypass) is doing two things to your installation. It's decreasing the efficiency of the coil and sending condensate into the ductwork. Moisture and mold go together and the wet duct also adds to what is often mistakenly called re-evaporation from the coil.


In your white paper, you seem to imply that when you say "excess water takes up free space and reduces the amount of free area for air to flow at an efficient (500 ft. per minute) velocity through the coil. It has the same effect as reducing the size of the coil….."

My logic is: If a coil is loaded with water, that water takes up space. If this water is not blown out of the coil, the space for air to travel though is reduced and the air going through that space is moving faster. Thus a greater bypass factor.


I would assume that the saturated coil would re-evaporate humidity when the discharge air sensor shuts the compressor off…. 

That’s logical.


What do the psychometrics look like under these conditions?  Are those conditions conducive to mold? 

I don’t have a clue about the psychometrics that might be conducive to mold growth!


What happens when and if the evaporator coil freezes, then thaws out when all zones are calling? 

Probably sprays a LOT of water into the ducts.


I would really appreciate and value your thoughts and scholarship on this topic. 

Thanks for the opportunity!


Danny Gough (housedoc)



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Dick's notes

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  • Hey Dick!

    Since I've been using the fan handler on furnaces, air conditioners, water source heat pumps, air handlers and regular heat pumps, things have been turning out great for our customers. On heat pumps the air is warmer and on gas furnaces it is just great. Everyone has been very pleased, which has greatly boosted customer's confidence in us. Best of all we can sell temperature controlled variable speed any time or all the time. We don't have to go to the manufactures so called variable speed.



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